If the headset supports both Bluetooth protocols, then the ringtone won't be heard using A2DP unless the manufacturer has done some special tweak.

You can block the speaker from getting the ringtone. Since the Bluetooth SCO (mono) takes control as soon as the phone starts ringing, you have to do this:1) Open SoundAbout2) Press "Alert behavior"3) Press "Bluetooth (mono) behavior"4) Press "Ringtones to"5) Choose "BT ringer (phone vibrate)" or "BT ringer (phone silent)"